1. Field of the Invention
The present invention relates to an X-ray computed tomographic apparatus in which a direct-drive (DD) motor is mounted to a gantry.
2. Description of the Related Art
In the past, X-ray computed tomographic apparatuses have been widely used in order to obtain a tomographic image of a subject. In most of current X-ray computed tomographic apparatuses, an X-ray tube and an X-ray detector are configured to continuously rotate around the subject. A gantry is provided with a motor rotationally driving an annular rotary frame mounted with the X-ray tube and the X-ray detector. In recent years, an X-ray computed tomographic apparatus has been distributed in which a direct-drive (DD) motor is used as the motor.
The direct-drive motor mainly includes a plurality of stator coils arranged in a fixed part in a circumferential shape and a plurality of rotor magnets arranged in the inside thereof so as to be opposed to the stator coils. A pulse signal is supplied from a servo amplifier to the stator coil at a frequency in accordance with a coil arrangement and a necessary speed. Accordingly, it is necessary to accurately adjust the polarity change timing of the pulse signal in accordance with a position of the rotor magnet with respect to the stator coil, that is, a distance between a center position of the stator coil and a center position of the rotor magnet and its direction in a stop state (hereinafter, referred to as a magnetic-pole position).
In the past, a hall IC or a photo sensor is used to detect the magnetic-pole position.
However, in a method using the hall IC, it is necessary to provide a hall IC disposed between a stator motor coil and a rotor magnet, a substrate mounted with the hall IC to extract a signal, a connector, and a cable. Additionally, since it is necessary to ensure some gap between the stator motor coil and the rotor magnet in order to mount the substrate to a position between the stator motor coil and the rotor magnet, the efficiency of the motor deteriorates. Additionally, it is necessary to consider a breakdown of respective electric components.
In a method using the photo sensor, since the number of electric components increases like the method using the hall IC, it is necessary to consider the breakdown. Additionally, since it is necessary to mount a cut plate for a photo sensor and to match an edge position of the cut plate with that of the magnet, the shape is limited. Since it is necessary to ensure a space in a body-axis direction in order to mount the photo sensor, the cut plate, there is a limitation in performing a mounting operation to the apparatus.